1. Field of the Invention
This invention relates in general to systems and methods for controlling semiconductor processing tools, and more particularly, to systems and methods for controlling heating by a processing tool by measuring current flow to the heat source of the tool.
2. Description of Related Art
Semiconductor fabrication uses a wider variety of processing tools, a large number of which heat semiconductor wafers. Rapid thermal annealing, in particular, has become one of the key process steps in the production of semiconductor devices. One of the most important issues during this fabrication process is the control of wafer temperature. Take dopant activation, for example. A rapid thermal annealing (RTA) temperature which is too low will cause insufficient alignment of the dopants in the crystal structure causing the channel to have high resistivity. A rapid thermal annealing temperature which is too high can, for example, cause dopant penetration into undesirable areas of the wafer. As such, precise control of the wafer temperature during the rapid thermal annealing process is directly related to the expected yields from a processing run.
To control the wafer temperature during RTA processing, an optical pyrometer measuring system is typically utilized which measures the back side radiation off the wafer at infrared wavelengths (typically 2.7 microns). This pyrometer output is used in a closed loop feedback system to control the pulse width modulation signal to the halogen lamps which heat the wafer during the rapid thermal annealing process. The backside radiation off various wafers, however, varies widely. This causes the pyrometric output signal used to measure the chamber temperature to possess a wider than desired variance over the useful temperature distribution. This problem causes a lack of repeatability from wafer to wafer at a given temperature.
Additional errors in pyrometric temperature measurement result from variations in the backside film thickness of the wafers. Attempts to overcome the limitations of pyrometry temperature measurement systems have included the use of dual wavelength techniques for providing additional control over temperature measurements. These techniques have been used experimentally. While they appear to improve the results for thin film systems, they offer little improvement for thicker films systems. Moreover, these techniques increase cost and complexity of the system.
Other methods have been used in an effort to overcome the limitations of pyrometer systems. One such method includes measuring two optical parameters in an effort to measure the emitted radiation directly and the emissivity directly. In this type of system, the reflectivity of the wafer is measured as a means to calculate the emissivity. While tighter control over wafer temperature can be obtained, this method is also more costly and complicated. As a result, it can be seen that there is a need for a less complicated and inexpensive way of controlling the chamber temperature during rapid thermal annealing process.